1. Field of the Invention
The invention relates to apparatus for grinding mineral products and similar hard materials with the aid of a medium agitated by a pin rotor, i.e. a rotor equipped with pins.
2. Description of the Prior Art
The technique of grinding or milling material with the aid of an agitated medium (Stirred Ball Milling) has been known for almost 60 years. The industrial breakthrough with this technique was made in 1948 in conjunction with pigment milling in the paint and laquer industries. The technique has been developed progressively over recent years and has been increasingly applied. As a result, many different types of mills in which milling is effected with an agitated medium have been proposed, as is evident, for instance, from an article in International Journal of Mineral Processing, 22 (1988), pages 431-444. One of these mills includes pin-agitator rotors by means of which the necessary milling energy is introduced by forced displacement of the grinding medium.
Due to the ability of the mills to grind or comminute material rapidly to very fine grain sizes, normally within a range of 1-10 microns, grinding with agitated media has been utilized to progressively greater extents with various types of material. Such fine grinding processes are thus employed in the production of fine grain products within the paint and laquer industry, the pharmacological industry, the electronic industry, the agrochemical industry, the food industry, the biotechnical industry, and the rubber, coal and energy industries. Coal-oil-mixtures and coal-water-suspensions are examples of this latter use. The technique of grinding with an agitated medium is also beginning to be applied in mineral technology. For instance, the technique is applied for grinding limestone, kaolin, gypsum, aluminium hydroxide and in the manufacture of paper fillers and paper coating materials. It is evident from this recitation that the majority of applications within the mineral field are concerned solely with "soft" material which is milled relatively easily. One such mill is described in U.S. Pat. No. 4,244 531 with particular reference to milling aluminium powder and cocoa powder, among other things.
In recent time, the use of this technique has been reported in South Africa for the recovery of the precious metal content of pyrites and roasted pyrites, wherein it is found that leaching of extremely small amounts of gold and other precious metals from these products is highly enhanced with particle fineness. It is found, however, that grinding for maximum recovery requires a grinding time of 6-8 hours. EP-A-0 267 170 described the re-grinding of a return product in the processing or enrichment of mineral products, wherein the return product is re-ground with the intention of crushing individual half-grains (i.e. in principle particles which contain both ore and gangue) and with the intention of separating valuable mineral. The regrinding process is carried out in a mill with the aid of an agitated grinding medium and the material is ground to a particle size smaller than K.sub.80 =100 microns. According to the publication, re-grinding is effected with a limited energy input.
The results of the research carried out in recent years show that the fineness of the ground material achieved when grinding with agitated grinding media is contingent solely on the specific energy supply, which can be expressed kwh/tonne of ground material. The results also show that the advantages afforded by this grinding technique over alternative techniques are greatly enhanced with increased fineness of the ground material, i.e. grinding with an agitated grinding medium becomes progressively more attractive with the fineness desired of the final product. Thus, a finer end product requires a higher specific energy input, i.e. a higher specific power input and/or longer grinding time. Obviously, an attempt to achieve greater fineness is initially made with a higher power input, so as not to negatively influence the productivity of the mills. Grinding times of 6-8 hours, as mentioned for instance in connection with pyrite grinding in South Africa, is naturally not as attractive, although in many cases necessary since an increase in power input would place unduly large demands on the ability of the mill to withstand a hard wearing environment, particularly when grinding such hard materials as ores. It has been found that the pin rotor of the mills is primarily subjected to wear much more quickly when the power input is increased. These pins normally consist of non-alloyed steel, although there have been tested steel pins provided with an external hard metal coating with the intention of increasing wear resistance. This hard metal protective coating may also have the form of a sleeve fitted onto the pin and screwed firmly thereto, or in the form of a surface hardened coating applied directly to the steel surface, as proposed in GB-A-1 197 582 for instance.
Hitherto, efforts to increase the power in mills which use pin rotors have failed because of the excessively rapid wear of the pins or because the pins loosen from the rotor, this drawback presenting a serious obstacle which in practice makes it impossible to use this technique for the purpose of processing ores.
There is a progressively increasing need for enabling the power input to be increased when grinding progressively harder materials, for the purpose of producing a progressively finer grinding product when grinding with a grinding medium which is agitated by a pin rotor.